1. Field of the Invention
An aspect of the present invention relates to a semiconductor device, and more particularly, to a CMOS (Complementary Metal Oxide Semiconductor) transistor and a fabrication method thereof.
2. Description of the Related Art
With the recent miniaturization of large scale integrated circuits, CMOS transistors are requested to achieve a thin gate insulating film. However, as the gate insulating film is made thinner and thinner, when a polysilicon film is used in a gate electrode, depletion of the gate electrode cannot be disregarded. For this reason, it has recently been investigated to use a metal electrode in the gate electrode, and in order to achieve a low threshold voltage in both a NMOS transistor and a PMOS transistor, there has been investigated a so-called dual gate structure where the gate electrode is formed using materials having different work functions.
Moreover, a proposal has been made to use a high-permittivity insulating film of, for example, Hf oxides, as the gate insulating film to thereby make the physical film thickness thicker, thus achieving an electrical thin film while suppressing a leak current. However, when the high-permittivity insulating film is used as the gate insulating film, in case of using polysilicon in the gate electrode, the polysilicon gate electrode may be depleted, and thus, the electrical film thickness increases. In this regard, it has been tried to use a combination of the high-permittivity gate insulating film and a metal gate.
As a high-permittivity gate insulating film of a next-generation MOSFET, an insulating film containing Hafnium (Hf) or Zirconium (Zr) is promising. However, in a CMOS transistor using a hafnium silicon oxynitride (HfSiON) film as the high-permittivity gate insulating film, because of a phenomenon called pinning, even when materials having different work functions are used, the flat band voltage (Vfb) cannot be adjusted to a band edge but is shifted to near a mid gap, thus making it impossible to achieve a low threshold voltage. In this regard, it has been found effective to use a cap film, especially containing lanthanoides in order to adjust the Vfb of a NMOSFET to the band edge (see JP-2002-270821-A, for example) However, forming the cap film in both NMOS and PMOS regions requires complicated and many steps including several repetitions of processes of film-forming, patterning, etching, and the like.